There are radio frequency identification (RFID) systems which include at least one RFID reader, also known as an interrogator, and a plurality of electronic transponders, also known as RFID tags. The RFID tags are typically passive, being energized by a broadcast radio frequency (RF) signal or power-up beam from the interrogator, but can also be active or semi-active, having an additional power source like a battery. The reader communicates with the tags by modulating commands and data onto the power-up beam (the so-called “forward link”), while the tags communicate with the reader by reflecting a varying amount of the energy from the power-up beam back to the reader, also known as “backscatter” (the so-called “return link”). The rules which define the parameters of the communication between the tags and reader are known as the air protocol. The protocol parameters typically include aspects such as encoding schemes, baud rates, command sequences, command formats and responses.
There are a large number of different RFID air protocols in use in the world today. These protocols can be broadly classified according to whether the reader initiates the communication between tag and reader, or whether the tag initiates the communication. If the reader initiates the communication, the protocol is called a “Reader Talks First” or RTF protocol. If the tag initiates the communication, the protocol is called a “Tag Talks First” or TTF protocol. An extreme case of a TTF protocol is the “Tag Talks Only” or TTO protocol, in which the reader never modulates the power-up beam to talk to the tags.
An example of an RTF protocol is the ultra high frequency (UHF) air protocol ISO/IEC 18000-63, one of the best known and most widely used air protocols today. ISO/IEC 18000-64 is an example of a TTO air protocol, while IP-X is an example of a TTF air protocol (which can also operate in a TTO mode). Although the rest of this discussion will use these passive UHF air protocols as examples, the classification into RTF, TTF or TTO is not limited to UHF air protocols, but is equally applicable at other carrier frequencies, such as at low frequency (LF) or high frequency (HF). It is also not only applicable to passive RFID, but also to semi-active, battery assisted or active tag systems.
RTF protocols such as ISO/IEC 18000-63 typically have complex algorithms for managing tag populations. These include complex schemes for selecting subsets of tag populations based on tag memory content or other aspects, and then inventorying and finally singulating tags. Once a tag has been singulated, i.e. a one-on-one communication channel between a specific tag and the reader has been opened, the reader can access the rest of tag memory, e.g. writing data to tag memory or reading data from tag memory. This access could possibly be preceded by security measures, such as exchange of access passwords or setting up a secured encrypted channel.
The complex but flexible RTF approach is theoretically capable of managing tag populations of various sizes efficiently. Large amounts of data can be handled, securely if needed. It has, however, a number of drawbacks:                The reader-driven RTF anti-collision process creates a large amount of interference. Reader transmissions are maybe 10 orders of magnitude more powerful than tag backscatter (maybe +36 dBm vs. maybe −80 dBm) and can interfere with tag backscatter over long ranges. As a result, readers cannot share spectrum channels or even operate in adjacent spectrum channels. When there are more readers to be deployed in proximity than the available spectrum channels, readers have to be time-multiplexed. This puts an upper limit on the total throughput that can be achieved at a single installation.        The RTF protocol has difficulty handling fast moving tagged objects. Movement speed is limited by the rate at which the reader initiates new inventory rounds (“polling rate”). In addition, spectrum regulations and available channel bandwidth limit the baud rate that can be used for the forward link, placing a further limit on the speed that tagged objects can move past an RTF reader.        
TTO protocols do not suffer from any of the above drawbacks. There is no reader modulation and therefore very little reader interference. Multiple readers can be operated simultaneously in proximity in the same spectrum channel. There is no polling since tags automatically transmit their ID and data as they enter the reader beam. Since there is no forward link, there is also no spectrum regulation induced limit on the baud rate. TTO protocols can therefore handle fast moving tagged objects much better than RTF protocols.
The claim invention proceeds upon the desirability of removing or at least improving the above mentioned problems relating to RTF protocols by proposing a method by which a TTO capability can be added with very little effort to an existing RTF protocol.